A Compact Hexagonal Structured Dual Band MIMO Antenna for ...

A Compact Hexagonal Structured Dual Band MIMO Antenna for Fixed

WiMAX Application

S.Oudayacoumar

M.Amudhan

Assistant professor ,Dept of ECE PG scholar, Dept of ECE

SMVEC ,Puducherry, India SMVEC,Puducherry,India

Abstract

A new compact multiple-input multiple-output (MIMO)

antenna with good isolation and compact size is

proposed. In this paper, the single port wimax antenna

and hexagonal structured MIMO antenna is studied

experimentally regarding return loss and VSWR. The

value of VSWR for both single port and MIMO antenna

is ≤ 2 in the frequency ranges from 1.8 GHz to 2.9 GHz

and from 4.5 GHz to 6.6 GHz. The single port wimax

antenna can produce two resonant frequencies having

return loss of -32 dB at 2.5 GHz and -42 dB at 5.6 GHz.

The proposed MIMO antenna resonates only at

2.5GHZ, but the return loss is so minimum when

compared to single port antenna. It is about -84dB

twice the time greater than the single port antenna. The

simulation results demonstrate the -10dB impedance

bandwidth at (1.8-2.9) GHz and (4.5-6.6 GHz). The

Hexagonal Structured Dual Band MIMO Antenna is

simulated using Ansoft HFSS and the same structure is

fabricated. This fabricated antenna is verified by good

agreement between simulated and measured results.

Keywords—Worldwide Interoperability for Microwave

Access (WiMAX), return loss, voltage standing wave

ratio (VSWR), radiation pattern, ultra wideband

(UWB),MIMO.

1. Introduction

In today's fast paced world, multiple-input

multiple-output (MIMO) technology made a great

breakthrough by satisfying the demand of higher

quality mobile communication services without using

any additional radio resources. The huge potential of

MIMO technique is evidenced by a rapid adoption into

the wireless standards, such as WLAN, LTE (Long

Term Evolution), and WiMAX. The multiple-input

multiple-output (MIMO) wireless communication

system has been extensively researched due to its

higher data transmission rate than the single-input

single-output (SISO) system in a rich multipath

environment.In 2002, the Federal Communications

Commission (FCC) allocated the ultra wideband

(UWB) frequency range from 3.1-10.6 GHz for

unlicensed UWB applications [1]. There are some

wireless communication applications which have

already occupied frequencies in the UWB band such as

the wireless local area network (WLAN) IEEE802.11a

and HIPERLAN/2 WLAN which operate at

(5.15-5.35) GHz and (5.725-5.825) GHz, respectively.

In addition, the use of the 2.5 to 2.7 GHz, 3.3 to

3.8 GHz and 5.25 to 5.85 GHz frequency band has been

limited by IEEE 802.16a/d/e.

WiMAX is a broadband wireless data

communications technology based around the IEEE

802.16 standard providing high speed data over a wide

area [2]. WiMAX is a technology for point to

multipoint wireless networking. WiMAX technology is

expected to meet the needs of a large variety of users

from those in developed nations wanting to install a

new high speed data network very cheaply without the

cost and time required to install a wired network, to

those in rural areas needing fast access where wired

solutions may not be viable because of the distances

and costs involved. Additionally it is being used for

mobile applications, proving high speed data to users

on the move.

Initially 802.16a was developed and launched,

but now it has been further refined. 802.16d or

802.16-2004 was released as a refined version of the

802.16a standard aimed at fixed applications. The

revision of the IEEE 802.16 standard falls into two

categories such as Fixed WiMAX and Mobile WiMAX.

Fixed WiMAX also called IEEE 802.16-2004, provides

for a fixed-line connection. Fixed WiMAX operates in

licensed bands (2.5 GHz and 3.5 GHz), and license free

band of 5.8GHz [3, 4]. Mobile WiMAX (IEEE

802.16e), allows mobile client machines to be

connected to the Internet. Mobile WiMAX opens the

doors to mobile phone use over IP, and even high-speed

mobile services.

Fixed WiMAX works under three different

bands. The low band frequency ranges from 2.5 GHz to

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2.8 GHz, the middle band frequency ranges from

3.2 GHz to 3.8 GHz and the high band frequency

ranges from 5.2 GHz to 5.8 GHz. Those antennas

should be smaller in size, less weight. The material is

also an important factor for designing an antenna, the

characteristics of the material should have minimum

loss, low cost and easily available [5, 6]. The printed

microstrip patch antenna has got the above

characteristic. This microstrip patch antenna consists of

a dielectric substrate [7], with a ground plane on the

other side. Conventional microstrip antennas in general

have a conducting patch printed on a grounded

microwave substrate [8].

A novel dual band[9] Hexagonal Structured

Antenna for IEEE 802.16d Fixed WiMAX system is

proposed which is printed on the FR4 substrate with a

dielectric constant of 4.4 and thickness of 1.6mm[10].

This antenna is designed and its parameters are

simulated using the electromagnetic (EM) simulator

called Ansoft High frequency Structured Simulator

(HFSS) package.

The paper is organized as follows: In Section

2, the single port wimax antenna is designed with

optimal geometric values. In section 3, the simulation

results and analysis are discussed. In section 4, the

hexagonal structured wimax MIMO antenna is

presented. In section 5, the simulation and analysis of

MIMO antenna is discussed. Section 6, concludes the

paper.

2. Geometry of Single port Wimax Antenna

The single port Wimax antenna geometry

structure is shown in Fig.1. The antenna has a compact

size of 62mm (L) x 58mm (W) x 1.6mm (H). The

substrate is made up of FR4 material which has an

relative permittivity (εr)=4.4. The radiating element of

the proposed antenna is composed of two hexagonal

structured elements which are separated by a distance

(D) as shown in Fig.1 (a) top view. The radiating

element is excited by a long microstrip line of length L1

which is connected to the two hexagonal structures.

Fig.1 (b) and Fig.1 (c) shows the bottom and side view

of the single port wimax antenna. The Single port

antenna geometric values are given in the Table I.

L and W represent the length and width of the proposed

antenna. L1 represents the distance between the feed

line and the radiating element. W1 represents the

position of feed line from the left axis. D denotes the

distance between the two radiating hexagonal elements

which is connected the horizontal strip of width 1mm.

The width of the feed line is about 1.5mm and its

impedance is about 50Ω to match it with the SMA

connector. The response of the proposed antenna varies

when the distance between the radiating hexagonal is

varied. In [11] the radius of the circle which is used to

unite the vertical feed line and the horizontal microstrip

is of 2.5mm.

(a)Top view

(b)Bottom view

Fig.1. Geometry of Proposed Antenna

In the proposed antenna the Hexagonal

structure shape which is present at its either side gives a

wider bandwidth [12] and the two rhombic slots [13] in

(c) Side view

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the hexagonal structure with the sides of length S2

decides the operating frequency. The dimensions of the

rhombic slots are tuned such that the operating

frequency of the proposed antenna falls at 2.5 GHz and

5.6 GHz (Fixed WiMAX). The circular strip which is

present at the canter couples the horizontal and the

vertical feed line which increases the flow of energy

through the vertical strip[14] to the hexagonal through

the coupled horizontal strip.

TABLE I

Optimal geometric values of the Antenna

3. Results and Discussion

3.1 Return Loss

The simulated result of return loss (S11) of the

proposed antenna is shown in Fig.2. It can be observed

that the proposed antenna exhibits dual band

characteristic in WiMAX frequency range i.e. this

antenna resonates at two frequency bands such as

2.5GHz and 5.6 GHz. The return loss of the proposed

antenna is about -42 dB at 5.6 GHz and -32 dB at

2.5GHz. The impedance Bandwidth of the proposed

antenna with respect to return loss is measured at

-10dB. The 2.5 GHz band operates at bandwidth of

1100 MHz i.e. 1.8 to 2.9 GHz and the other 5.6 GHz

band operates at the bandwidth of 2100 MHz i.e. 4.5 to

6.6GHz. When the distance D between the two

hexagonal structures is varied [15] as 30mm and 35 mm

the return loss varies as shown in the Fig.2. The return

loss is about -42dB at D=30mm whereas the return loss

is -30dB at D=35mm, which shows that the proposed

antenna with the distance D=30mm has better return

loss and all the further parameter analysis are based on

D=30mm.

Fig.2. Simulated return loss for various ground

plane length.

3.2 VSWR

As shown in the Fig.3 the proposed antenna

had achieved VSWR of value ≤ 2 at two frequency

bands of frequency i.e 1.8 GHz to 2.9 GHz and

from 4.5 GHz to 6.6 GHz for 2.5 GHz and 5.6 GHz

respectively. The value of VSWR is exactly one at

the operating frequencies 2.5GHz and 5.6 GHz, this

shows that the proposed antenna suits well for the

WiMAX services especially for IEEE 802.16-2004

or 802.16d.

3.3 Radiation Pattern

The proposed antenna is directional in nature

as it radiates symmetrically as shown in Fig.4 (a) and

Fig.4 (b). The radiation characteristic [16] of the novel

dual band antenna is simulated for both H-Plane and

E-Plane. The far field radiation pattern is being checked

for two frequencies 2.5 GHz and 5.6 GHz. At 2.5GHz,

the E-Plane and H-Plane characteristic is symmetric in

nature and it radiates at two direction equally at φ=0o

(E-Plane) and φ=90o (H-Plane) as shown in Fig.4.

(a).The radiation characteristic at 5.6 GHz is shown in

Fig.4 (b) which is symmetric in nature.

S.No Notations Description Dimensions

in mm

1. D

Length of the strip

line connecting

hexagonal

30

2. S Distance between

Ground and feed 3.2

3. H Substrate Thickness 1.6

4. L1 Feed line length 37.1

5. W1 Distance of feed

line from X-axis 30.3

6. L2 Length of hexagonal 16.8

7. W2 Width of hexagonal 10.4

8. S1 Side of the triangle 10.4

9. S2 Side of the

Diamond 7

10. L Substrate Length 62

11. L3 Length of Ground

plane 62

12. W4 Width of Feed line 1.5

13. W Substrate Width 58

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Fig.3. Simulated VSWR of proposed antenna

The E-Plane which is radiated at 5.6 GHz is symmetric

which radiates in both the sides equally [17] but it has

two minor lobs at φ=90o (H-Plane).

The comparison of proposed dual band

hexagonal structured antenna with existing [3] wide

band umbrella antenna is shown in Table II.

3.4 Fabrication of Proposed Work

The proposed hexagonal structured dual band

antenna has been fabricated on the FR4 substrate which

is of thickness 1.6mm. The front and bottom view of

the fabricated antenna is shown in Fig.5 (a) and

Fig.5 (b).

TABLE II

Comparison of Existing WiMAX antenna and

proposed antenna

Parameters Existing

Antenna[3] Proposed antenna

Size 20x30x1.6mm3 62x58x1.6mm

3

Material Neltec (εr=2.6) FR4 Epoxy(εr=4.4)

Substrate

Thickness 1.6mm 1.6mm

Return loss -30dB at 5.6 GHz -32dB at 2.5 GHz

and -42dB at 5.6

GHz

VSWR Less than 2

Approximately

equal to one at

operating

frequencies

No. of

Bands

Single Band at

WiMAX Double Band

Design

Complexity

Very difficult to

fabricate such a

small antenna Easy to fabricate

Finally, the designed hexagonal structured

dual band antenna is measured with an Agilent N5230A

vector network analyzer. The simulated and measured

S-parameters and VSWR are in an excellent agreement

as observed in Fig.5.1 (a) and Fig.5.1 (b).

Fig.4 (b). 5.6 GHz Gain (dBi) for φ=0o (E-

Plane) and

φ=90o (H-Plane)

Fig.4 (a). 2.5 GHz Gain (dBi) for φ=0o (E-

Plane) and

φ=90o (H-Plane)

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Fig.5 (a) Front view

Fig. 5 (b) Bottom view

Fig.5 Prototype of the proposed antenna

Fig.5.1 (a) Simulated and measured S-parameters of

the proposed antenna.

Fig.5.1 (b) Simulated and measured VSWR of the

proposed antenna.

4. Geometry of Hexagonal structured

Wimax MIMO Antenna

The geometry of the proposed Hexagonal

Wimax MIMO Antenna is shown in Fig.6.1. The

antenna has a compact size of 124mmx58mmx1.6mm.

The substrate is made up of FR4 material which has a

relative permittivity (εr) =4.4. As shown in Fig 6.1 (a)

top view two similar structures of hexagonal is

designed. One Structure is placed at 0o and other

structure in placed at angle of 90o

[7]. Both the

structures are placed on same substrate which is made

of FR4 Epoxy. Two different ground plane in made for

the two structures separately and each one is excited

with a long microstrip line of width 1.5mm.There is no

physical contact between two structures and thus an

array of antenna element is made. Each SMA connected

to the strip lines is having the same impedance of about

50Ω. L5 and W5 represent the length and width of the

proposed antenna. L1 represents the distance between

the feed line and the radiating element. D1 denotes the

distance between the two radiating hexagonal elements

which is connected the horizontal strip of width 1mm as

shown in Fig.1.

The width of the feed line is about 1.5mm and

its impedance is about 50Ω to match it with the SMA

connector. The response of the proposed antenna varies

when the distance between the radiating hexagonal is

varied. The radius of the circle which is used to unite

the vertical feed line and the horizontal microstrip is of

2.5mm. The two antenna elements are kept in such a

way that it gives better return loss. The elements which

are placed at 90o difference give better response.

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(a)Top View

(b)Bottom View

(c) Side View

Fig.6.1 Geometry of Proposed MIMO Antenna

The antenna response changes from dual band

to single band because of placing of multiple antenna

elements. The antenna resonates only at 2.5GHz, but

the return loss value is so minimum when compared to

single element antenna. It is about -84dB twice the time

greater than single element antenna.

5. Results and Discussion of MIMO

Antenna

5.1 Return Loss

The simulated result of return loss (S12) of the

proposed antenna is shown in Fig.6.2. It can be

observed that the proposed antenna exhibits 2.5GHz

band characteristic in WiMAX frequency range. The

return loss of the proposed antenna is about -84 dB at

2.5 GHz. The impedance Bandwidth of the proposed

antenna with respect to return loss is measured at

-10dB.The 2.5 GHz band operates at bandwidth of 1100

MHz i.e. 1.8 -2.9 GHz.

As we discussed earlier, the two antennas are

integrated on the same dielectric substrate to construct a

co-located antenna system in such a position relative to

each other that there is maximum isolation between

them. For MIMO systems, the mutual coupling between

the antennas is one of the important parameters to

evaluate the performance of the antenna system. It is

clear from the Fig 6.2 the mutual coupling between

(S12) and (S21) is always less than -15dB in the

frequency range of interest therefore, it ensures that two

elements on proposed co-located antenna system can

operate simultaneously.

Fig.6.2 Return loss for proposed MIMO antenna

5.2 VSWR

As shown in the Fig.6.3 the proposed antenna

had achieved VSWR of value ≤ 2 at two frequency

bands of frequency i.e. 1.8 GHz to 2.9 GHz and from

4.5 GHz to 6.6 GHz for 2.5 GHz and 5.6 GHz

respectively. The value of VSWR is exactly one at the

operating frequencies 2.5GHz, this shows that the

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Fig.6.3. the VSWR of proposed MIMO antenna

Proposed antenna suits well for the WiMAX services

especially for IEEE 802.16-2004 or 802.16d.

5.3 Radiation Pattern

The proposed antenna is directional in nature

as it radiates symmetrically as shown in Fig 6.4. The

radiation characteristic of the novel dual band antenna

is simulated for both H-Plane and E-Plane. The far field

radiation pattern is being checked for two frequencies

2.5 GHz and 5.6GHZ. At 2.5 GHz, the E-Plane and H-

Plane characteristic is symmetric in nature and it

radiates at two direction equally at φ=0o (E-Plane) and

φ=90o (H-Plane) as shown in Fig.6.4.

Fig.6.4 2.5 GHz Gain (dBi) for φ=0o (E-Plane) and

φ=90o (H-Plane)

5. Conclusion

Wideband operations of the novel hexagonal

structured dual band MIMO antenna have been

demonstrated. Constructed prototype is studied at

WiMAX operation at two bands in IEEE 802.16(d)

Fixed band at 2.5 GHz and 5.6 GHz bands. This

antenna has two bands at 2.5GHz and 5.6 GHz whereas

when an antenna array (MIMO) it resonates only at

2.5GHz with better response than other types. The

Proposed antenna has a low profile and is easily able to

feed by microstrip line with SMA of 50Ω. The

proposed antenna has the characteristics of dual band

response with good return loss above 30dB and

radiation pattern at E-Plane and H-Plane. The proposed

antenna is a simple and effective feeding structure in

design, has adequate operational bandwidth at WiMAX

operating range.

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